Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system (CNS) and mediates neurotransmission in most excitatory synapses. Three classes of glutamate-gated ion channel receptors—α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainite, and N-methyl-D-aspartate (NMDA) receptors—can transduce postsynaptic signals. Among them, NMDA receptors are the most abundant and ubiquitously distributed throughout the brain. Therefore, they are fundamental to excitatory neurotransmission and critical for the maintenance of normal CNS function. However, excessive glutamate overstimulates NMDA receptors, leading to increased intracellular calcium and excitotoxicity (Kemp J A, McKernan R M. 2002. NMDA receptor pathways as drug targets. Nat Neurosci 5: 1039-1042.). It is by the glutamate-dependent mechanism that neurons die in various CNS disorders, including brain ischemia, epilepsy, and Alzheimer's disease. The role of NMDA receptors in excitotoxicity has driven the search for antagonists as neuroprotective agents.
On the other hand, NMDA receptor activity is essential for normal neuronal function. Potential neuroprotective agents that block virtually all NMDA receptor activity lead to unacceptable clinical side effects (drowsiness, hallucination, and even coma) because they block normal NMDA receptor activity (Palmer G C. 2001. Neuroprotection by NMDA receptor antagonists in a variety of neuropathologies. Curr Drug Targets 2: 241-271.). For this reason, many previous NMDA receptor antagonists have failed advanced clinical trials in a number of neurodegenerative disorders. In contrast, some studies have shown that the adamantane derivative memantine can block excessive NMDA receptor activity without disrupting normal activity and shows promise in clinical applications (Chen H S, Lipton S A. 2006. The chemical biology of clinically tolerated NMDA receptor antagonists. J Neurochem 97: 1611-1626.). Memantine exerts its pharmacological effects through its action as a low-affinity, uncompetitive open-channel blocker. Memantine has unique blocking sites in channel pores, and this subtle difference between memantine and other traditional NMDA receptor antagonists may explain many advantageous properties of memantine action. In fact, in normal conditions, the excitatory postsynaptic current resulting from physiological activation of NMDA receptors is mostly preserved. In excitotoxic conditions, when prolonged activation of NMDA receptors occurs, memantine becomes a very effective blocker. In essence, the pharmacological effects of memantine are most obvious under pathological conditions, and it maintains the normal functions of receptors, thus relatively sparing synaptic transmission and preserving long-term potentiation and maintaining physiological function (Chen H S, Wang Y F, Rayudu P V, Edgecomb P, Neill J C, Segal M M, Lipton S A, Jensen F E. 1998a. Neuroprotective concentrations of the N-methyl-D-aspartate open-channel blocker memantine are effective without cytoplasmic vacuolation following post-ischemic administration and do not block maze learning or long-term potentiation. Neuroscience 86: 1121-1132.). In fact, memantine has been used clinically with an excellent safety record for more than 20 years in Europe to treat Parkinson's disease, spasticity, convulsions, vascular dementia, and Alzheimer's disease.
NMDA receptors are involved in neuronal survival and maturation (Marshall J, Dolan B M, Garcia E P, Sathe S, Tang X, Mao Z, Blair L A. 2003. Calcium channel and NMDA receptor activities differentially regulate nuclear C/EBPbeta levels to control neuronal survival. Neuron 39: 625-639.), neuronal migration (Komuro H, Rakic P. 1993. Modulation of neuronal migration by NMDA receptors. Science 260: 95-97.; Kihara M, Yoshioka H, Hirai K, Hasegawa K, Kizaki Z, Sawada T. 2002. Stimulation of N-methyl-D-aspartate (NMDA) receptors inhibits neuronal migration in embryonic cerebral cortex: a tissue culture study. Brain Res Dev Brain Res 138: 195-198.), induction of long-term potentiation (Bliss T V, Collingridge G L. 1993. A synaptic model of memory: long-term potentiation in the hippocampus. Nature 361: 31-39.; Zhao J P, Phillips M A, Constantine-Paton M. 2006. Long-term potentiation in the juvenile superior colliculus requires simultaneous activation of NMDA receptors and L-type Ca2+channels and reflects addition of newly functional synapses. J Neurosci 26: 12647-12655.), formation of sensory maps (Simon D K, Prusky G T, O'Leary D D, Constantine-Paton M. 1992. N-methyl-D-aspartate receptor antagonists disrupt the formation of a mammalian neural map. Proc Natl Acad Sci USA 89: 10593-10597.), and neurodegeneration (Cull-Candy S, Brickley S, Farrant M. 2001. NMDA receptor subunits: diversity, development and disease. Curr Opin Neurobiol 11: 327-335.; Zieminska E, Stafiej A, Lazarewicz J W. 2003. Role of group I metabotropic glutamate receptors and NMDA receptors in homocysteine-evoked acute neurodegeneration of cultured cerebellar granule neurones. Neurochem Int 43: 481-492.). Under normal conditions of synaptic transmission, the NMDA receptor channel is gated by extracellular Mg2+ sitting in the channel and only activated for brief periods. This brief opening of the NMDA receptors allows Ca2+ (and other cations) to move into the cell for the subsequent physiological functions. Under pathological conditions, however, overactivation of the receptor relieves the Mg2+ block and causes an excessive amount of Ca2+ influx into the nerve cell, which in turn triggers a variety of processes that can lead to necrosis, apoptosis, or dendritic/synaptic damage. These detrimental processes include: (1) Ca2+ overload of mitochondria, resulting in oxygen free-radical formation, activation of caspases, and release of apoptosis-inducing factor; (2) Ca2+-dependent activation of neuronal NOS, leading to increased NO production and formation of toxic peroxynitrite (ONOO—) and S-nitrosylated glyceraldehyde-3-phosphate dehydrogenase (GAPDH); and (3) stimulation of mitogen-activated protein kinase p38, which activates transcription factors that can go into the nucleus to influence neuronal injury and apoptosis (Chen H S, Lipton S A. 2006. The chemical biology of clinically tolerated NMDA receptor antagonists. J Neurochem 97: 1611-1626.).
In excitotoxic conditions, mitochondrial dysfunction associated with loss of Ca2+ homeostasis and enhanced cellular oxidative stress plays a major role in cell damage (Frandsen A, Schousboe A. 1993. Excitatory amino acid-mediated cytotoxicity and calcium homeostasis in cultured neurons. J Neurochem 60: 1202-1211.; Jacquard C, Trioulier Y, Cosker F, Escartin C, Bizat N, Hantraye P, Cancela J M, Bonvento G, Brouillet E. 2006. Brain mitochondrial defects amplify intracellular [Ca2+] rise and neurodegeneration but not Ca2+ entry during NMDA receptor activation. FASEB J 20: 1021-1023.). Under these circumstances, stimulation of ionotropic glutamate receptors causes massive Ca2+ entry and is highly involved in the process of neuronal death. Energy depletion and increased oxidative damage to several synaptic proteins such as Na+, K+-ATPase may result in loss of local Ca2+ homeostasis and membrane depolarization. As a consequence, synaptic degeneration follows. In addition, Ca2+ is known to activate several intracellular enzymes, such as phospholipase A2, nitric oxide synthase, xanthine dehydrogenase, calcineurin, and endonucleases. Many of these enzymes can elicit generation of endogenous ROS (Rego A C, Oliveira C R. 2003. Mitochondrial dysfunction and reactive oxygen species in excitotoxicity and apoptosis: implications for the pathogenesis of neurodegenerative diseases. Neurochem Res 28: 1563-1574.). Moreover, an increase in mitochondrial Ca2+ itself can also promote ROS generation (Kowaltowski A J, Castilho R F, Vercesi A E. 1995. Ca2+-induced mitochondrial membrane permeabilization: role of coenzyme Q redox state. Am J Physiol 269: C141-147.), and intracellular Ca2+ overload associated with excitotoxicity can induce both apoptosis and necrosis (Ankarcrona M, Dypbukt J M, Bonfoco E, Zhivotovsky B, Orrenius S, Lipton S A, Nicotera P. 1995. Glutamate-induced neuronal death: a succession of necrosis or apoptosis depending on mitochondrial function. Neuron 15: 961-973.). Therefore, elevation of intracellular Ca2+ and increased ROS production are the major causes of neuronal death in excitotoxicity.
The potential for the consumption of tea or tea polyphenols to prevent or ameliorate chronic disease is becoming the subject of considerable scientific investigation. Although a number of mechanisms have been proposed to explain the beneficial effects of tea in different models of chronic disease, the radical scavenging and antioxidant properties of tea polyphenols are frequently cited as important contributors (Liang Y C, Lin-Shiau S Y, Chen C F, Lin J K. 1997. Suppression of extracellular signals and cell proliferation through EGF receptor binding by (−)-epigallocatechin gallate in human A431 epidermoid carcinoma cells. J Cell Biochem 67: 55-65.; Lin Y L, Lin J K. 1997. (−)-Epigallocatechin-3-gallate blocks the induction of nitric oxide synthase by down-regulating lipopolysaccharide-induced activity of transcription factor nuclear factor-kappaB. Mol Pharmacol 52: 465-472.). Evidence supporting an antioxidant function for tea polyphenols is mainly derived from assays of their antioxidant activity in vitro. Recently, the in vivo evidence that tea polyphenols are acting directly or indirectly as antioxidants has been progressively expanded (Umemura T, Kai S, Hasegawa R, Kanki K, Kitamura Y, Nishikawa A, Hirose M. 2003. Prevention of dual promoting effects of pentachlorophenol, an environmental pollutant, on diethylnitrosamine-induced hepato- and cholangiocarcinogenesis in mice by green tea infusion. Carcinogenesis 24: 1105-1109.). In this aspect, animal studies can offer a unique opportunity to assess the contribution of the antioxidant properties of tea polyphenols to the physiological effects of tea administration in different models of oxidative stress (Frei B, Higdon J V. 2003. Antioxidant activity of tea polyphenols in vivo: evidence from animal studies. J Nutr 133: 3275S-3284S.).